Photoelectrochemical cells (PECs) can directly utilize photogenerated electron-hole pairs in semiconductor electrodes for fuel production as nature does through photosynthesis. In a typical PEC, fuels are formed by reduction reactions at the cathode which consume photoexcited electrons. Examples include the reduction of water to H2 and the reduction of CO2 to carbon-based fuels such as methanol and methane. In order to complete the circuit, oxidation reactions occur at the anode, consuming photogenerated holes. Typically, water oxidation to O2 is used as the anode reaction, which is environmentally benign and does not require additional species in the electrolyte. Another critical role of water oxidation as the anode reaction for a sustainable PEC operation is the generation of H+ (2H2O →O2+4H+) to offset the H+ consumption accompanied by the cathode reaction reducing water or CO2. However, water oxidation is not a kinetically favored reaction, and its product, O2, is not of significant value. Therefore, identifying an anode reaction that has more favorable kinetics and can generate value added chemicals would be beneficial for increasing the overall efficiency and utility of PECs.
The production of building block chemicals as well as fuels using renewable energy sources is critical in order to be completely independent from fossil fuels. To achieve this goal, as well as to address the aforementioned issues, oxidatively producing building block organic molecules using biomass-derived intermediates as alternative anode reactions of PECs is an exciting and desirable strategy. Among the various biomass resources and intermediates, 5-hydroxymethylfurfural (HMF) derived from C6 monosaccharides, which are obtained by depolymerization of cellulosic biomasses, is considered a key platform molecule that can generate various industrially important molecules via further conversion. For example, one of its oxidation products, 2,5-furandicarboxylic acid (FDCA), can serve as a monomer to produce important polymeric materials such as polyethylene terephthalate and poly(ethylene 2,5-furandicarboxylate) and is considered a possible replacement for terephthalic acid. Another oxidation product of HMF, 2,5-diformylfuran (DFF), has the typical chemical properties of an aldehyde. Its applications include use in the synthesis of pharmaceuticals, antifungal agents, macrocyclic ligands, and organic conductors, as an intermediate and monomer for the synthesis of polymers and as a key building block for porous organic frameworks.
Most previous studies of the conversion of HMF to FDCA utilized aerobic oxidation using heterogeneous catalysts. Typically, the reaction is performed in an alkaline aqueous solution (pH≧13) under high pressure O2 or air (3-20 bar), usually at elevated temperatures (30-130° C.) using precious metals (e.g. Au, Pd, and Pt or their bimetallic alloys) as catalysts. An alternative approach to aerobic oxidation is electrochemical oxidation where the oxidation is driven by the electrochemical potential applied to the electrode, which eliminates the use of O2 or other chemical oxidants. Electrochemical oxidation can also offer the advantage of controlling the oxidation potential and monitoring the reaction rate by the current, which may provide additional mechanistic insights. However, only a few reports have been published for electrochemical oxidation of HMF to FDCA to date. A study by Strasser and co-workers probed the feasibility of electrochemical oxidation of HMF using a Pt electrode in a pH 10 aqueous solution and found that a fraction of HMF could be converted to 2,5-diformylfuran (DFF) but conversion of HMF to FDCA was negligible. They noted that water oxidation was the major competing reaction and likely limited the Faradaic efficiency (FE) for HMF oxidation. A more recent study by Li and co-workers reported the use of carbon black supported precious metal nanoparticles (Au/C, Pd/C Pd2Au/C, PdAu2/C) and demonstrated that the use of PdAu2 alloy nanoparticles significantly enhanced the conversion of HMF to FDCA (83% yield). However, FDCA was obtained with other oxidation intermediates such as 5-hydroxymethyl-2-furan-carboxylic acid (HMFCA) and complete conversion to FDCA was not reported in their study.